On one hand, it's a huge invasion of privacy, and something we should never do.

On the other hand, it's really not too complex of a concept. If your boyfriend or girlfriend isn't doing anything shady on his or her phone, there's zero prerogative to try and keep things from you. If your significant other is prone to hiding his or her phone, or the contents of said phone from you, it's probably because he or she does have something to hide.